Production of carbenium ions from carbenes by protonation

Diphenyl-, aryl-a-siloxyand %-furany1 carbenes can be protonated by even weak proton donors such as alcohols (by a bimolecular or by a pre-association mechanism involving more than one alcohol molecule) to yield the corresponding carbenium ions, whose spectra and electrophilic patterns of behavior are described. INTRODUCTION Proton transfer reactions to and from heteroatoms (= "normal" Bronsted acids and bases, with rate constants for proton transfer usually at or near the diffusion controlled limit) are different from those to and from carbon atoms.' In aqueous solution, the latter typically undergo proton transfer reactions more slowly than do their heteroatom counterparts and are believed to do so directly rather than through solvent molecules. This difference in behavior is probably due to differences in changes in hybridization, in heavy atom bond lengths and angles, in hydrogen-bonding ability, in (de)solvation in the transition state, and changes in delocalization of charge. So far, nearly all of the studies related to the acidbase chemistry at carbon atoms relate to carbanions and their conjugate acids.' Little is known about the acid/base chemistry of divalent carbon atoms, carbenes. However, carbenes can exhibit a considerable reactivity even towards weak proton donors such as alcohols or water, the reaction leading to carbenium ions, whose spectra and nucleophilic reactivities have been determined, using the technique of laser flash photolysis (LFP).2-4 For these cases, the mechanism of the formal insertion of (nucleophilic) carbenes into 0-H bonds, which has been discussed in terms of a) initial formation of an oxonium ylide followed by a 1,2-hydrogen shift, b) a concerted mechanism, or C) by proton transfer (Scheme l), has thus been decided upon in favor of reaction C: